merging two objects for sculpting
You select both objects and press Ctrl+J, to make them one object. But if you sculpt, you'll easily get the separate parts appear to be dividing up where they're not joined by their vertices. The best thing to do would be reduce the number of edge flows (temporarily) so you can cut an edge around the body where they meet the legs, delete the faces that are inside the legs, press Ctrl+J to join them, then merge vertices so you have one continuous surface.  Hopefully you have an UN-applied smoothing modifier on which you can delete to simplify the geometry.